I think it was more like 3/8 with a 14" piece of wood...maybe a 1/4".
That is a lot.
One of the reasons I built our second teardrop was that our first –-a Hunter commercial 'drop-- didn't have a roof-vent fan. It had a computer fan in the bulkhead. It moved the air from the cabin into the galley which had two side vents. It was noisy and didn't move enough air. That's how important ventilation in a teardrop is.
It's (almost) rare for us to run the fan where we camp. But we
always have the vent at least cracked. When it's warm, it's all the way open and it does a great job pulling air in from the windows while our body heat convects out the top. When it's hot, that's when we run the fan for a half hour or so to get the cabin cooled off in the evening.
If it were me, I'd explore fabricating a couple of wedges to form a 14" flat spot. Nearly 200 square inches moves a lot of hot air.
Tony
p.s. Gorgeous build you have going.